The checkers and stressers should be composable without special cases; this
patch tries to address that while refactoring out some old cruft.
Namely,
* Single stresser/checker for a tester; built from composition
* Composite stresser via comma-separated list of stressers
* Split stressers into separate files
* Removed v2 only flags and special cases
* Rate limiter shared among key stresser and leases stresser
* Composite checker is now concurrent
* Stresser can return a Checker to check its invariants
* Each lease checker only operates on a single lease stresser
